Design for review

Discussion in 'Boat Design' started by PsiPhi, Jan 12, 2022 at 2:16 AM.

  1. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Short Version:
    Working on a design, for my own amusement and possible building, of a "Swallows and Amazons" style sailing dinghy, and was hoping that people who know better than I could pass judgement on it.
    Currently just a basic hull design done in Delftship, but will draw up properly once I know if I am on the right track.

    The criteria I set myself is:
    • Have something of a Swallows and Amazons look and feel - make me feel like I am 14 years old and chasing pirates up Lake Windemere or the Amazon River.
    • approx 13ft long, 5ft beam
    • a boat you sit in, not on
    • sedate and stable for an old guy with dogey knees
    • economical, and not to difficult, to build
    • Single lug sail
    • Centreboard
    ScarabLines.jpg
     
  2. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    At first glance, looks reasonable from here. Delftship software usually fairs the hull lines, whereas your drawing shows abrupt line changes at every junction, hopefully a proper Deftship model will be forthcoming.
     
  3. PsiPhi
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    PsiPhi Newbie

    Ah, my bad. I had put some creases in while looking at frame shapes.
    Scarab2.jpg
     
  4. wet feet
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    wet feet Senior Member

    What sort of displacement are you aiming for?I raise the matter because it looks like a low sort of amount,but the amount of hull above the water seems above average for a 13 foot boat.Which might mean it will be a bit hard to row if the wind drops.Have you thought of making a cardboard or balsa model to look at or push around a trough of water?
     
  5. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    I am aiming to carry one or two people (c80-90 kilo each) plus the weight of the boat (aprox 5 sheets 9mm and and one sheet 12mm, plus framing and rig), and picnic food. I don't have the skills to calculate that yet but most of the 13ft designs I looked at say they carry 2~4 people for day sailing.

    I started the design process based on a very handly "Getting Started in Boats" insert in WoodenBoat magazine, it was a How to Design Your Own Boat one. Basically it said to find a half a dozen similar boats with similar intended use, look at their basic stats, and 'smoosh' them together - that is an intellegent adopting of various dimensions, not a blind average. I looked at a number of boats on the Selway-Fisher site, but also got the plans for the YM Junior, which I considered building too.

    I was intending a cardboard or flexi-ply model, hadn't thought of pushing it around the bathtub though, but I will now. I hadn't considered rowing either, which I should have. When I make my cardboard model I will make a scaled sized person to see how the seating and rowing might work. I will have to have a motor as local water is tidal.

    The high freeboard is to satisfy my desire for a boat you sit in, not on. A boarding ladder over the stern is also on the plan.
     
  6. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    I am not a designer, so take my comments in that light.
    I look at that double chine and 'v' bottom cross section and I see something very close to a peapod. A very round bilged rowboat that, while moves nicely through the water, is tippy. You might consider taking the 'v' out of the bottom unless you're using ballast.
     
  7. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    SailboatData.com - NORFOLK URCHIN Sailboat https://sailboatdata.com/sailboat/norfolk-urchin
    PWhQ1cs9sh2EXHC9y2d0t3MU80x1HDGAyqCDp5H7.jpeg
    Norfolk Urchin
    LOA: 13.00 ft / 3.96 m
    Beam: 5.17 ft / 1.58 m
    Weight of boat (including all equipment): 255kgs
    (This weight was taken from a for sale ad. Sailboatdata.com didn't give a displacement for this boat.)

    Ad the weight of your two crew to get the displacement.

    You can get a rough calculation of your designed displacement, if your design software doesn't do it for you, by averaging the area below the waterline, of two adjacent cross sections and multiplying that by the distance between sections. Add all the areas between sections up to get the cubic displacement, the volume of water the boat is expected to displace. If the weight of that volume of water matches the material weight of the boat, your waterline is in the right place.
     
  8. PsiPhi
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    PsiPhi Newbie

    I'm not a designer either Will, but it is fun to try to be. I didn't want a flat bottomed design as we have tidal currents and short chop a lot of the time, was hoping this shape would help to cut through that, and avoid slapping, without being too deep in the water
     
  9. PsiPhi
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    PsiPhi Newbie

    It does, comes up at 280 kg at the designed draft. Kennith Gibbs YM Junior puts it's dead weight at 12o kg, which seems light compared to the Urchin. I think I need to do some weight calculations.
     
  10. messabout
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    messabout Senior Member

    I doubt that you need to use ply a thick as you have mentioned. A boat of that sort will be plenty tough with 6mm and 9 mm ply. On the other hand, if you anticipate extremely rough use, then use the heavier ply.

    I agree that you may have excessive freeboard in the design. Sixteen inches at midships is probably enough to sustain you in choppy water. The design you have drawn, is as previously noted, going to be a bit tender because of its rounded bottom. A simple vee bottom will deal with a moderate chop and also be much easier to build. It would also have somewhat better initial stability.
     
    Will Gilmore likes this.
  11. PsiPhi
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    PsiPhi Newbie

    I did some quick, very guesswork, calculations on wieght, based on the thicker ply and using dubious online calculators, and the total came up very close to the weight of the Urchin mentioned above.

    I think I will definately look at the thinner ply as, although we get a bit of chop, it's wind-over-tide sort of chop, not large, and I am not planning on rough weather sailing, not while still a novice.
     
  12. PsiPhi
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    PsiPhi Newbie

    Once I have my cardboard model I will look again at the freeboard, maybe it is too high.

    I do have an altierer motive for the round bottom too. Inspired not only by Arthur Ransome, but also by the writings of Chas Stock, and his journeys in Shoal Waters, I am considering, if all goes well, to one day move up to a boat like his, and Kennith Gibbs's "Senior" is closer to Shoal Waters than a lot of other boats, indeed it is one of the designs Stock considered. My design above leans heavily on Gibb's "Junior", this is sort of a trial run to see how I might go building, and sailing, a boat like that. The reason for designing my own is just because I like to endulge my creative side, and so many desgins, good designs, I look at think they would be idea if only ...
     
  13. PsiPhi
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    PsiPhi Newbie

    But I don't have a spell-checker :D
     
  14. wet feet
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    wet feet Senior Member

    I understand the desire to feel the security of sitting inside the boat rather than on top of it.On the other hand it needs to be balanced against the extra windage that is unavoidable with such a hull.After a few further calculations,it may be that the keel needs to move a little deeper to gain a bit of displacement and if the thwarts move down a touch too,it might be a better compromise and all boats are a bundle of compromises.

    One other thing that catches my eye is the stem profile-it looks a bit too rounded between the topside chine and the chine just above the waterline.Which may indicate that the panels might not be developable.Does Delftship allow you to determine if the panels can be developed?It can be a real fight to force ply greater than about 4mm thick to a non-conical surface shape.
     

  15. mc_rash
    Joined: Aug 2020
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    mc_rash Junior Member

    Another point may be the stern, I guess this design won't be a planing hull and operate mostly in displacement speeds. Move the transom slightly upwards so it is not immersed anymore to avoid extra drag produced by an immersed transom. It's just a tiny immersed area that maybe could be ignored from the resistance view, but its also just a tiny amount of space which "misses" if it's removed and thus actually does not miss.
     
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